OK things are looking great. From what I've read, not experienced, the yellowing of the cotyledon (first funny round set) means the plant is ducking some of the nutrients out. The cotyledon hold the basic nutes it needs to get started. The sunshine mix#4 is great but you have to b careful which one you get. Some have some nutrients added and one does not have anything but some mycos really. ...can u post a pic of the front of the bag of sunshine your using? I didn't seem to have any issue. I got the small yellow and black promotional bag. Is the one with NO nutrients. I begin my feeding schedule on day ten. I used advanced nutrients, not super familiar with gen hydro but I assume to start feeding the same time, day 10ish.
Now the other possibility which im leaning towards is that you grabbed the mix with nutes added, and the autos are more sensitive to stuff like that. Good job on the added perlite for air in the soil! I couldn't remember to do that until this third grow lol. But yeah could u get that pic by chance? Or if the bag is gone send me a link to the exact bag u purchased?